In spite of its vast gold and silver reserves, the United States still relies on imports for its precious metals. According to Government figures, more than 50% of the gold and 45% of the silver consumed domestically are imported from foreign countries. Obviously, this compounds the national economic difficulties with respect to the trade deficit. It is, therefore, important that an economic, efficient process be found for the recovery of gold and silver from low grade ores.
Many processes have been studied for the extraction of precious metals such as gold and silver from low grade ores. Cyanidation is a commonly employed process in which the gold and silver in crushed ore is dissolved in a dilute solution of sodium or calcium cyanide and a small amount of lime in the presence of oxygen, with the gold dissolving in the form of an aurocyanide complex. Recently, activated carbon and charcoal have been used to adsorb and recover the precious metals from the dilute solutions of alkaline cyanide or from other solutions, including sulfite and halide solutions, resulting from hydrometallurgical treatment of the ores. Such a process has also been used to adsorb and recover the precious metals in concentrates, wastes, tailings, and slimes. (See, for example, Heinen et al, "Processing Gold Ores Using Heap Leach-Carbon Adsorption Methods," U.S BuMines IC 8770 (1978); and "Carbon-in-Pulp Gold Recovery Process," J. S. Afr. Min. Eng. 90 (4152) (1979)).
The adsorbed precious metals must still be eluted or desorbed before the precious metals can be prepared from the eluate by other processes. These other processes include an electrowinning of the desorbed gold solution to obtain the metal values. Another process includes the chemical precipitation of the precious metals. A chemical stripping process disclosing the desorption of loaded activated carbon is disclosed in the Heinen U.S Pat. No. 4,208,378. This patent also discloses the use of electrolysis to win metal values from a stripping solution. Another process is the Zadra process (see BuMines RI 4843 (1952)) in which a sodium hydroxide, sodium cyanide mixture (NaOH-NaCN) at 95.degree. C. is used to elute the gold adsorbed on carbon. However, this process takes 50 to 100 hours to elute 300 ounces of gold per ton adsorbed. The precious metals are then electrowon from the cyanide eluate.
Although recent improvements have been made in the composition of eluates and conditions to shorten desorbing time, the metals adsorption-desorption and metal preparation for producing the precious metals is a two-step process. The precious metals adsorbed on the carbon must first be desorbed and secondly must be produced from the eluates by other means. Other methods of obtaining the precious metals from the loaded carbon has included burning the carbon, but that process is very expensive.
Processes which attempt to overcome the problem of economically obtaining the precious metals from a leaching solution are disclosed in a number of U.S. patents. The Hazen U.S. Pat. No. 3,767,543 discloses an electrolytic process for removing copper directly from a chloride leach solution. The aforementioned Heinen et al patent discloses in its discussion of the background of the invention a somewhat analogous process for removing gold. The Loretto U.S. Pat. No. 3,926,752 and the Fraser U.S. Pat. No. 4,204,922 disclose other methods of recovering metals from ore through the use of electrolysis of a solution. The Hazen and Loretto patents disclose processes relating to the metal copper, and the Fraser et al patent is more general in that it relates to the use of electrolysis of any metal in the cationic series such as copper, zinc, lead, nickel, tin, antimony, molybdenum, and silver. But again, these patents disclose processes which have multiple steps and thus are lengthy and expensive. Further, many of these processes disclose the use of complicated and expensive electrolysis with cells having diaphrams.
The prior art lacks an efficient and simple method for desorbing and producing precious metals from previously loaded activated carbon. In fact, none of the prior art processes discloses or discusses the feasibility of desorbing and simultaneously producing precious metals from loaded carbon or charcoal.